Process for lining a surface using an organic film

ABSTRACT

The present invention relates to a method for cladding a simple or complex surface, electrically conducting or semiconducting, by means of an organic film from at least one precursor of said organic film, characterized in that the cladding of the surface by the organic film is carried out by electro-initiated grafting of said, at least one, precursor of said surface by applying at least one potential sweep on this surface carried out in such a way that at any point of said surface the maximum potential of each potential sweep, in absolute value and relative to a reference electrode, is greater than or equal to the value of the potential (v bloc ) from which the curves of a graph expressing the quantity of electro-grafted precursor on a surface identical to said surface in function of the number of potential sweeps are all superposed and independent of this v bloc  potential.

This application is a national phase application of PCT Application No.PCT/FR2003/050035 filed Aug. 25, 2003, which claims the benefit ofFrench Patent Application No. 0210568, filed Aug. 26, 2002, which areboth hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to the field of organic surface coatings,the said coatings being in the form of organic films. In particular, itis related to a process for making a lining on a surface by anelectrically initiated reaction using an organic film with a uniformthickness, even at a scale less than or equal to 1 micrometer, on aconducting or semiconducting surface.

In many applications, it is important to obtain organic linings orcoatings with a uniform thickness, particularly at a scale less than 1micrometer, and in particular when this coating acts as a protectivecoating, for example an anti-corrosion, biocompatible coating, etc., oras an electrically insulating or conducting coating, an opticalabsorbing coating, a coating on biochips or chemical sensors, etc., inwhich the properties resulting from this coating are almost directlyrelated to its thickness. Any local non-uniformity in the thickness isthen the cause of a local defect in the required performance.

PRIOR ART

Processes are known for deposition of organic linings that can operateon almost all types of surfaces, and therefore particularly onelectrically conducting and semiconducting surfaces. For example, thereare physical and chemical vapour phase deposition (CVD, PVD, etc.)processes, and centrifuging and spin-coating processes.

However, PVD and CVD depend on the existence of suitable precursors toobtain the reactive vapour necessary to make the deposit. Therefore,only some types of coatings are possible, which makes it difficult tooptimise functions to be performed by the coating. Furthermore,particularly in microelectronics, it is found that PVD is sensitive tothe three-dimensional topology of the surface and particularly to pointeffects, particularly on structures with large depth to width aspectratios. This sensitivity is probably the result of the greaterreactivity of areas with geometric projections and causes more markedthickness non-uniformities if the structures are thin (<0.13 μm).

Thickness non-uniformities of films are also observed when the PVD orthe CVD have to be used in combination with a masking device, resultingfrom mask edge effects. Finally, the thickness check of organicdepositions by CVD is now insufficient for ultra-thin coatings in theindustrial domain, particularly for thicknesses of less than a fewmicrons, which makes this range of thicknesses even more inaccessible.

Edge effects are also observed for deposition by centrifuging (“spincoating”). This process enables good uniformity of thicknesses as longas the liquid flow deposited under the effect of the centrifugal forceremains laminar. In general, this is not the case on the edges ofsurfaces nor vertically in line with rough paving that causes flowturbulence (“Eckman spirals”). Furthermore, for ultra-thin films, it hasbeen observed that evaporation of the solvent during application of theprocess increases the viscosity of the fluid (often locallynon-uniformly) and causes thickness non-uniformities. Furthermore,devices making use of these processes are complex since they require acontrol over saturating vapour pressures, the temperature and even thehumidity.

Solvent annealing processes for organic coatings deposited by“spin-coating” have been developed to overcome the above-mentioneddisadvantages. However, they require an additional step with asignificant cost.

Other processes based on electrochemistry have been developed to obtainorganic coatings with an adjustable thickness. Unlike previousprocesses, these processes directly use the electrical properties ofconducting and semiconducting surfaces and are only applicable on thesesurfaces. For example, it has been known for many years that polymerswith fillers on electrically conducting or semiconducting surfaces canbe electrically deposited. For example, it is also known how to doelectroplating of metals, or electro-polymerisation of conductingpolymer precursor monomers such as pyrrole, aniline, thiophene, EDOT,etc. What these reactions have in common is that they are electricallydriven reactions, in other words they do not continue unless there is anelectrical current passing in the electrical circuit in which thesurface to be treated is one of the electrodes. A distinction should bemade between these reactions and electrically initiated reactions, inwhich only one step (the initial step) is electrochemical, and generatesa coupled chemistry independent of the current.

However, these processes have two significant handicaps thatsignificantly reduce their use in microelectronics, and more generallyin the manufacture of organic coatings on surfaces made fromsemiconducting materials:

Progress of electrically driven reactions depends on the electricalpotential and/or the current being maintained in the electrolysis cellthroughout the film growth period. Therefore the resulting coatingthickness depends on the integral current charge that passed through thecircuit.

These electrically driven reactions are insufficient to achieve filmthickness uniformities within a few tens of nanometers on surface areasof several square centimetres, for example as is the case frequently inmicroelectronics. If two electrically conducting or semiconducting areasare electrically connected in series through an impedance, then not allof the electrical potential applied to one will be transmittedidentically to the other, despite the connection between the twosurfaces; there is a resistive drop between the two areas. Thisresistive drop implies that the apparent potentials applied to the twosurfaces are different, and therefore that the electrical currents thatpass through them are different, for equal areas. For equal electrolysistimes, the charges that passed through each area will be different, andthe film thicknesses obtained using an electrically driven reaction willalso be different.

In summary, electrically driven reactions result in coatings that areabove all faithful to the topology of resistive drops—related to theprocess and the device for implementing it—and not to the geometrictopology of the initial surface.

Furthermore, no process based on electrically driven reactions is usedat the present time to obtain organic coatings when it is important tobe able to achieve uniform thicknesses to within a few tens ofnanometers on surface areas of a few square centimetres, for example asis the case in microelectronics.

Considering the current difficulty in making surfaces with nonon-uniformity in the surface resistance, and perfect electrochemicalcells without any non-uniform current distribution, there is a real needto have organic film deposition processes that level the effects of aresistive drop between one point and another on a given surface.

This need is particularly important in all application fields in whichsemiconducting materials are used, for example in micro-electronics, inmicrosystems such as sensors, micromachines, etc., since these materialsare precisely characterised by the fact that their surface is notequipotential, and that there is always a resistive drop between any twodifferent points on this surface.

Thus, it is found that the efficiency of processes now available to makethe deposition of an organic coating on a conducting or semiconductingsurface with good control over the thickness, are closely related to thesurface topography, for example to the roughness for non-electrochemicalprocesses and to the topography of resistive losses for electrochemicalprocesses.

PRESENTATION OF THE INVENTION

The present invention provides a solution to the various problems inprior art mentioned above, by providing a process capable of quickly andreproducibly obtaining organic films with a uniform thickness, even forfilm thicknesses of less than 1 μm, on an arbitrarily-shaped supportsurface, and therefore in particular regardless of its topography.

The process used for the present invention is a process for lining asimple or complex, electrically conducting or semiconducting surface,using an organic film, starting from at least one precursor of the saidorganic film, characterised in that the surface is lined by the organicfilm by electrically initiated grafting of the said at least oneprecursor on the said surface by application on this surface of at leastone potential scan made such that, at all points on the said surface,the absolute value of the maximum potential of each potential scan withrespect to a reference electrode is greater than or equal to the minimumscanning potential value (V_(block)), and the curves in a graphexpressing the quantity of precursor electrically grafted on a surfaceidentical to the said surface as a function of the number of potentialscans are all superposed on this minimum scanning potential valueV_(block) and are independent of it.

Various measurements such as the real quantity in mol/cm² (concentrationper unit area) or the thickness of the resulting organic film (for amaximum grafting rate) can be used to evaluate the quantity of theelectrically grafted precursor. Depending on the case, some of thesemeasurements will be easier to carry out than others, depending on thechemical nature of the precursors and the organic films finallyobtained, and it would be good to choose the easiest, for example thethickness measurement using profilometric method, ellipsometry, atomicstrength microscopy or tunnel effect microscopy.

According to one particular embodiment of this invention, the organicfilm may be an organic polymer film and the monomer may be anelectro-active precursor monomer of the said organic polymer film. Inthis particular embodiment, the electrically initiated reaction is thenobviously an electrically initiated polymerisation of the said at leastone precursor monomer. In this embodiment, the graph may be a graphexpressing the thickness of the said organic polymer film as a functionof the number of potential scans.

In general, electrically initiated grafting reactions of this invention,hereinafter referred to as electrically initiated reactions, shall beconsidered separately from electrically driven reactions that areelectrochemically initiated reactions but for which progress cannotcontinue unless the electrical potential and/or the current ismaintained in the electrolysis cell. In other words, they areelectrochemical reactions generating coupled chemistry containing atleast one electrochemical reaction. As described above, the thickness ofthe coating obtained by the electrically driven reaction depends on theintegral charge of the current that passed through the circuit, andtherefore the local resistive drop.

The process according to this invention does not depend on theseparameters. In fact, only the electro-initiation step, also called theelectro-priming step, depends on the electrical current. In particular,this initiation depends on electro-activation of the precursor of theorganic film, for example leading to the formation of species that havebeen oxidised or reduced starting from the precursor(s), and aretherefore subsequently only the source of molecular or macro-molecularspecies without any significant electro-activity in the electrodepotential ranges used.

For example, polymer films grafted using the particular embodiment ofthe process according to this invention obtained by grafting ofelectrically activated monomers on conducting or semiconductingsurfaces, are obtained by electro-initiation of the polymerisationreaction starting from the surface followed by growth of the chains,monomer by monomer. Grafted chains are allowed to grow by purelychemical polymerisation, in other words independently of polarisation ofthe conducting surface from which grafting was done. Therefore, it isquite clear that the potential protocol plays a distinct role from thecomposition of the solution. Once growth of the chains has begun, growthof the film is managed by the composition of the solution: all chainswill be the same length since the solution facing the surface is thesame for all growing chains. Therefore, the thickness of the organicfilm grafted on the surface is adjusted by adjusting the grafting ratioon the surface, and this thickness is the same everywhere when thisgrafting ratio is maximum.

Therefore according to the invention, if a series of coatings is made onsuccessive surface areas that are all strictly identical, and ifidentical appropriate operating conditions are imposed on each surface,a maximum grafting ratio can be achieved every time on every surface byusing potentials greater than V_(block), the surface thicknesses can bechanged on different surfaces simply by changing the composition of thesolution.

The process according to the invention can include reactions based oncompetition to terminate the growth of chains: these reactions interruptthe growth of chains and therefore contribute to fixing the maximumthickness of the film when the chains are in brush form using theprocess according to the invention. The radicalar inhibitors and activesite transfer agents in radicalar polymerisation reactions (particularlymembers of redox couples), protons (and by extension protic molecules)and electrophiles and particularly cations, nuleophiles and particularlyanions, are candidate compounds for acting as a termination agent forelectrically initiated reactions for this invention in order to adjustthe thickness control if necessary.

The electrically initiated reactions that could be used in the processaccording to this invention as claimed are reactions in which theelectro-initiation can correspond to a reduction of a monomer, or anoxidation of a monomer using an electrical current or in which theelectrically reduced monomer is the initiator of anionic polymerisationreactions, or it can correspond to cationic polymerisation reactionswhen the electro-initiation applies to oxidation of a monomer, on thesurface and in solution. It may also consist of reactions initiatedusing an electro-active mediator; once oxidised or reduced, thisactivated mediator acts as a usually radicalar polymerisation initiator.Candidate mediators include diazonium, phosphonium, sulfonium, iodoniumsalts, peroxodisulfates, sulfamates, metallic ions and complexes ofthem, etc.

According to the particular embodiment of this invention describedabove, the at least one electrically activatable precursor monomer ofthe organic film could for example be a vinyl monomer. In the case of avinyl monomer, it may advantageously be chosen from the group formedfrom vinyl monomers such as acrylonitrile, methacrylonitrile, methylmethacrylate, ethyl methacrylate, butyl methacrylate, propylmethacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate,glycidyl methacrylate, acrylamides and particularly amino-ethyl, propyl,butyl, pentyl and hexyl methacrylamides, cyanoacrylates, di-methacrylateglycol polyethylene, acrylic acid, methacrylic acid, styrene,parachloro-styrene, N-vinyl pyrrolidone, 4-vinyl pyridine, vinylhalides, acryloyl chloride, methacryloyl chloride and derivatives ofthem.

Analogues of these vinyl monomers obtained by coupling or modificationto one of these monomers with any molecule or macromolecule can also beused. This molecule or macromolecule may for example be chosen from thegroup composed of a polymer such as glycol polyethylene; a nitrogenbase, for example such as adenosine or its derivatives, for example suchas 3-deaza adenosine; or a trace nucleotide, for example such as a probesequence used on a DNA chip; a peptide, for example such as a prion; aprotein such as an enzyme, an anti-body, etc.; a fatty acid such aslinoleic acid; a glucide, for example such as glucose; a polysaccharidethat may or may not be modified such as dextrane and modified dextranes;cellulose and its derivatives; chitosane and its derivatives; etc. Inthis case, the precursor is a vinyl monomer coupled to this molecule ormacromolecule.

Depending on the nature of the reaction that allowed the coupling, itmay also conversely consist of molecules or macromolecules carryingseveral activated vinyl groups, and particularly those mentioned in theabove list.

When several precursor monomers are used in the process according tothis invention, it may consist of a mix of several vinyl monomers forexample chosen from among the above mentioned vinyl monomers. Ingeneral, it may consist of polymers, the growth of which was initiatedby an electro-active initiator with ionic reduction or oxidationproducts. In the case in which a mediator was used, the growth of thepolymers used is initiated by the electro-active mediator, withradicalar reduction or oxidation products.

The process according to this invention may also use electro-graftingreactions of cyclic monomers such as epoxides, lactones, for example□-caprolactone, etc. that are cleavable by nucleophilic or electrophilicattack. The principle of these reactions is the same as for vinylmonomers, except that the polymer film growth takes place by openingcycles.

Electro-grafting reactions of diazonium, sulfonium, phosphonium,iodonium, ammonium salts, alcohols, thiols, etc., can also be used inthe process according to this invention. These are molecules that whenreduced produce radicals that are self-adsorbed (chemical absorption)directly onto the surface and do not create any growth (passivation).For example, this is the case of processes used to make very smallthicknesses close to a molecular monolayer since there is no growth ofpolymer type chains. The blocking phenomenon mentioned in thisdescription is not the phenomenon that would block growth of a speciesthat would eventually hinder the growth of other chains formed later,but is directly the phenomenon of metallic sites through which thecurrent can pass.

The process according to this invention can also useelectro-polymerisation reactions initiated for example by peracids,thiolates, diazonium, sulfonium, phosphonium, iodonium salts or othercompounds for which the reduction or oxidation product is apolymerisation primer radical in the presence of monomers that can bepolymerised by radicalar methods such as vinyl polymers like thosementioned above, for example acrylonitrile, etc.

Processes according to which polymers are obtained tonically can also beused, for example activated vinyl polymers, cyclic molecules cleavableby nucleophilic or electrophilic attack initiated by anion or cationradicals of compounds such as naphthalenes, metallic salts and othercompounds for which the anion or cation radical is a polymerisationprimer.

A simple surface according to the meaning of this invention means asingle piece surface without any roughness, usually with a smoothsurface or a surface with a controlled roughness, particularly at thescale of the thickness to be achieved. A complex surface according tothe meaning of this invention means a surface composed of differentparts that can be distinguished or are distinct as a result of theircomposition and/or their shape, connected to each other by conducting orsemiconducting links or structured or unstructured surfaces withdepressed, etched or embossed parts, or glued parts or several of theseparts, and surfaces located on different parts of the support that areor are not connected, on which the film must be grafted using theprocess according to this invention.

The process according to this invention enables genuinely fine controlof the uniformity of the thickness of the organic film or coatingdeposited by electrically initiated reactions enabling the production ofuniform, ultra-thin organic linings or coatings less than 1 μm thick, inother words at scales that are very difficult to achieve industriallyusing processes according to prior art mentioned above on simple orcomplex surfaces.

Unexpectedly, the absolute value of the maximum potential of eachpotential scan with respect to a reference electrode, greater than orequal to the minimum potential value V_(block) using the processaccording to this invention gives almost maximum occupancy, usually morethan 50% of the conducting or semiconducting surface by the at least oneelectrically grafted monomer even in a first potential scan, and morethan 80% or even 90% after only a few scans. The inventors of thisinvention have observed that unexpectedly, two to five potential scansaccording to this invention are usually enough to achieve maximumoccupancy of the surface. This maximum occupancy situation is called the<<blocking>> situation. Growth of the grafted chains starting fromelectrically grafted monomers on the surface takes place by purelychemical polymerisation, in other words independently of thepolarisation of the conducting surface that caused grafting. Sinceoccupancy is maximum, the polymer chains are densely grafted onto thesupport since they are almost upright on the surface and form a “brush”.The consequence is that the film thickness is similar to the length ofthe extended chains, self-regulated and uniform.

Therefore, the inventors of this invention were the first to have madeorganic polymer films with a uniform thickness even at molecular scale,using the process according to this invention.

Obviously, the number of potential scans to obtain maximum surfaceoccupancy depends on operating conditions, the type of surface and thetype of electrodeposited organic polymer film. For example, it may be N,where N is a positive integer number, and 1≦N≦15. This example is notlimitative, but in general it is sufficient to obtain a quasi-maximum ormaximum occupancy of the surface by the precursor monomer(s).

Obviously, the graph showing the quantity of electrically graftedprecursors as a function of the number of scans according to thisinvention is preferably determined under the same operationalphysicochemical conditions as will be used for electro-grafting usingthe process according to the invention.

By way of illustration, according to one particular embodiment of thepresent invention, the monomer(s) and the solvent(s) required forelectro-grafting a polymer film according to the method of the inventionwill be the same as those utilized for determining the V_(bloc) by meansof the graphic. In this case, determination of V_(bloc) is realized by agraphic giving the thickness of the organic polymer film as a functionof the number of scans under the same physiochemical operationalconditions as that used for the electro-initiated grafting according tothe method of the invention.

According to the present invention, scanning of potentials can be acontinuous or discontinuous, sinus or in segments. For example, it canbe a scan under voltammetric conditions or polarization by multiplesegments. Polarization by multiple segment makes it possible to make asupplementarily adjustment of the relationship between the polarizationtime relative to the idle time, even if it is generally observed that itis possible to obtain a result similar to that under voltammetriccondtions using a lower number of scans.

The method of the present invention makes it also possible to achieve amaximum occupation rate of the surface, whether it is simple or complex,by the precursor; for example, the monomer precursor of an organicpolymer film, in virtue of the application of this scan of potentials atevery point of said surface conforming to the method of the presentinvention.

The present inventors are thus the very first to resolve these technicalproblems of the prior art connected with the effects of ohmic drops dueto non-homogeneities in surface resistance by evening out the effectsbetween one surface point and another, whether simple or complex, in thecontext of the present invention, in virtue of the method of the presentinvention.

The method of the present invention finds very interesting applicationin particular for the following surfaces:

Surfaces of perfectly controlled texture but comprised of or comprisingsemi-conductor materials and the area is very large in advance of thetexture or the patterns that have been engineered. This is the case, forexample, of the wafers of silicon, for example, in microelectronics, inmanufacturing coatings for the copper interconnection. In thisapplication, it is necessary to produce deposit material having lowdielectric permittivity (“low k dielectrics”), low thickness, in generalless than 500 nm, having a thickness control in the order of severaltens, that is a decade of nanometers on disks of 200 or 300 millimetersin diameter. Any electrochemical desposit presumes placement of anelectrical contact. Frequently, this electrical contact is realized onthe periphery of the wafer and there is an ohmic drop between theperipheral electrical contact and the center of the wafer. The method ofthe present invention also make it possible to obtain an organic film ofuniform thickness, even at thicknesses of less than μm on this type ofsurface.

The surfaces of the doped semi-conductor materials can have dopinginhomogeneities that can result in different ohmic drops between thecurrent carrying contact and different points of the surface. This canbe, for example, the case of wafers, for example, of silicon havingdoping zones and supplied electrically either in the periphery (corona),on on the back surface; in other words, for example, by means of acontact where the back fact of the wafer has been metallized, wholly orin part. The method of the present invention also makes it possible toobtain an organic film of uniform thickness, even at thicknesses of lessthan μm on this type of surface. The surfaces of the dopedsemi-conductor materials can have doping inhomogeneities that can resultin different ohmic drops between the current carrying contact anddifferent points of the surface. This can be, for example, the case ofwafers, for example, of silicon having doping zones and suppliedelectrically either in the periphery (corona), on the back surface; inother words, for example, by means of a contact where the back fact ofthe wafer has been metallized, wholly or in part. The method of thepresent invention also makes it possible to obtain an organic film ofuniform thickness, even at thicknesses of less than μm on this type ofsurface. The surfaces of the conductors or the semi-conductors on whichthe etchings have been made. When it is attempted to carry out depositsby electrolytic means of the prior art; in other words, by anelectro-tracking reaction over such surfaces; the counter-electrode isgenerally a planar surface, which does not have—more or less—a structureas complex as the surface itself. When the surface of the object isplaced facing the counter-electrode, the object/counter-electrodedistance varied from one point to another from the object, which resultsin some points not having the same ohmic drop, because they do notexperience the same volume of electrolytic solution between the twoelectrodes and thus, they are not brought to exactly the same potential.This is the case, for example, of etched wafers that can have wideetchings of several hundreds of nanometers, for which it is necessary toprovide organic coatings that conform as much as possible to thetopology of the etching. The attempt was made in the prior art tominimize this ohmic drop effect by using—for example, for the metaldeposit—solutions of elevated ionic strength. However, the recipe wasonly partially sufficing on the microelectronic scale and it wasgenerally necessary to complexity at some time the topology of thecounter-electrode in order to perfect the homogenization of the lines ofcurrent arriving on the surface to be treated. The method of the presentinvention makes it possible to overcome these drawbacks of the prior artand to obtain an organic film of uniform thickness, even at thicknessesof less than μm also in this type of surface.

The etched conducting or semi-conducting surfaces and whose etchingshave locally projecting geometries. These zones are the source of “pointeffect” that concentrate the field lines and are thus the source of avery local variation of the apparent ohmic drop. Most often, thisvariation is the source of a local increase in current, which has theeffect of locally augmenting the quantity of material deposited (bosses)and thus of impairing conformity of the coating by the homogeneity ofthe thickness. The method of the present invention also makes itpossible to obtain an organic film of uniform thickness, even atthicknesses of less than μm on this type of surface.

The present invention is applicable, for example, in any method oforganic functionalization of a surface; that is to say, in any methodthat comprises equipping a surface with a substrate or with a support byan organic film.

It is applicable, for example, in the field of Microsystems in general,such as detectors, micro-machines, etc., in which it is necessary,especially for reasons of costs, to collectively functionalize printedcircuits, when they are still on their original wafer-type support, bydepositing organic layers for lithography for a coating (chip scalepackage) etc., or for any other organic functionalization of thesurface, for example for adhesion for the polymer flip-chip, electricalinsulation, anti-adhesion of biological molecules, etc.

It also finds application in the realization of an insulating layer or abarrier layer in the Microsystems, such as the aforementioned, forexample, in microelectronics.

The thicknesses of the organic layers to be deposited in the differentapplications of the present invention can be of several hundreds ofnanometers to several microns or tens of microns for a waver surface,whose general dimensions are 200 or 300 millimeters.

The present invention also finds application in manufacturing biochips,for example, in the prior methods for collectively addressing moleculessuch as biomolecules, for example. In fact, for example, on protruding(flip-chip) sensor supports (wafer), distant component elements, forexample, gold spots, must sometimes be covered with organic coatings ofidentical thicknesses in order to make possible assembly of a secondcomponent without subsidence. The method of the present invention makesit possible to provide organic film polymers forming such organiccoatings and making possible assemblies of a second component withoutsubsidence.

The present invention finds application also in microelectronics wherethe surfacing can be reduced to a single zone that covers the totalityof the wafer. The thicknesses can then be of the order of a hundrednanometers or less, with a control that must be of several % on thescale of an object of 200 to 300 millimeters in diameter. The method ofthe present invention makes possible such a control.

The present invention also finds a plethora of applications in the fieldof optics, where the uniformity of the deposit is directly linked to theperformances of the macroscopic object. The specialist in the fieldknows well, for example, that in the field of biochips for fluorescentdetection and, more generally, by optical detection (generation of a sumfrequency, generation of second harmonic, etc.), the thickness of theorganic coating that serves primarily in capturing labeledoligonucleotides sequences is critical in the extinction of thefluorescent signal characterizing the diagnosis of hybridization:whatever the nature of this coating, the physics of the phenomenondemands thicknesses in the order of several tens of nanometers with aprecision in the order of 10 to 20%, for example 60 (10 nanometers for aCy3 type fluorophore), 80 (10 nanometers for a type CY5 type fluorophorelabel) for optimizing the signal to noise ratio. The same studiesdemonstrate an almost total extinction of the fluorescence signal if thethickness of the oligonucleotide fixation coating approaches a hundrednanometers. Miniaturization of the biochip supports is currently indevelopment but the spots of these supports have still dimensionscharacteristically greater than several tens or several hundreds ofmicrons. The present invention makes it possible to obtain the filmthicknesses required in these fields of optics. It thus makes itpossible to optimize the performances for novel generations ofminiaturized biochips.

The same physics leads obviously to the same constraints in the field ofoptics in general, of optoelectronics and photovoltaics. The presentinvention also responds to this type of constraint.

In addition, the present invention is applicable in the opticalperfection of fashion accessories, where the surface aspects areassociated with thicknesses in the order of visible wavelengths, but lowrelative to the shape of the object.

Other features and advantages will become apparent to the specialist inthe art on reading the description and the following examples givenillustratively but non-limitingly with reference to the appendeddrawings.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention resides in the grafting and thegrowth of organic films by means of electro-initiated polymerizationreactions. These are electrochemical reactions, one of whose products atleast is absorbed by physiosorption or chemisorption on the surface,this product can itself be implicated in a chemistry coupled with thetransfer of charge and, in particular, a chemistry that induces thegrowth of the organic film on the surface where the electro-initiationoccurred.

The particularity of this coupled chemistry is that its kinetics and/orits thermodynamics are not correlated to the electrical current thataccompanies the electro-initiation. These are chemical reactions andautonomous; in other words, non-electrochemical, whose parameters ofadvancement or interruption can be linked, for example, to theconcentration of the electro-activated species, to temperature, to thenature of the solvent, or even to the presence of a particular additivein the solution, but where the electrical current remains a parameterthat does not intervene directly in the thickness of the coating or theorganic film finally obtained.

Appended FIGS. 1 a) and b) represent the electrochemical reactionmechanisms used in an illustrative exemplary embodiment of the method ofthe present invention, when the precursor monomer of the organic polymeris acrylonitrile. Electro-grafting is done under cathodic polarization,the growth of the grafted chains is done by anionic polymerization.

The measured electrical current corresponds to manufacture of the anionradical adsorbe on the surface (S) by electronic transfer in FIG. 1 a).As this reaction diagram shows, only part of the radical anions thuscreated are going to have a sufficient life-span to be the initiators ofa polymerization reaction. The other part of these radicals is going todesorb in order to produce polymer in solution, thus liberating metalsites on which other monomers are going to be capable of being adsorbedin order to be reduced in their turn (FIG. 1 b).

This teaches the absence of direct correlation that can exist betweenthe voltammetric current and obtaining the grafting in the method of thepresent invention.

Generally, the voltammetric current diminishes on each scan, whichcorresponds to an increasingly progessive occupation of the metal sitesof the surface (S) by grafted polymer chains. This current ends bycanceling itself almost completely for a sufficient number of scans whenthe rate of occupation of the sites of the initial surface is such thatthere is no longer sufficient space available for the growth of a newchain. In this case, any residual current that can be present, even ifit is ordinarily very weak, corresponds largely to the step of FIG. 1 b)of the reaction diagram: grafting, inasmuch as it attains its maturity;that is, the maximal rate of chains per unit of surface has beenreached.

It is now important to examine what occurs with the chains of growinggrafted polymer: in the step of FIG. 1 a) of the reaction diagram, it isfound that growth of the chains is identical at all points at which itdevelops in any anionic polymerization, that it may otherwise beelectro-initiated or not.

This growth is essentially guided by the concentration of monomer aswell as of polymerization inhibitors. The short length of the chainsobtained by electo-grafting suggests that there is a certain impairmentbetween the adjacent growth extremities and that the kinietics ofpropagation is probably less than that which could be observed due toinitiation in solution. Whatever it may be, and even if the detailedmechanism linked to the growth of the grafted polymer chains is notcompletely elucidated, it can be captured diagrammatically that thelengths of the electro-grafted chains is not linked to the stage ofelectro-grafting itself: each grafted monomer occasioning at the startof the chain consumed one electron and one alone and the electrochemicalprotocol no longer had control of the propagation reaction. An essentialconsequence of the method of the present invention is that it is thuspossible to regulate the thickness with sufficient precision byproducing a more or less elevated rate of occupation of the sites: whenthe chains are “thinly” grafted on the support; they have the tendencyto lie on the surface, the measured thickness of the film obtained isthin and probably not uniform, at least at the molecular level.

When the polymer chains are densely grafted on the support; in otherwords under conditions of scanning of potential according to the methodof the present invention, that are almost erect on the surface and forma “brush” and the thickness of the film is close to the length of theextended chains.

Thus, for an electrolytic solution of a given composition, the higherthe rate of coverage of the surface that conditions the thicknessobtained according to the method of the present invention and not thegrowth of the chains themselves. Now, as has been describedhereinbefore, the rate of coverage can be gradually increased up to alimit level, where the growths of chains are prevented by excessiveloading on the surface. As this coverage rate conditions the thickness,the thickness of the electro-grafted coatings according to the method ofthe present invention is rather “autoregulated”.

The method of the present invention utilizes a monomer precursor of theorganic polymer. Once activated, that is reduced or oxidized, thismonomer becomes the source of a chemistry leading to the production ofmolecules or macromolecules that themselves impair the production oftheir kind, up to the point of blocking the entire process. Once theprocess is blocked, the electro-active monomer may continue to bereduced or oxidized, but the chemistry coupled to the transfer of chargeis limited to the formation of soluble species or no longer induce themodifications of the first molecular layer of the precursor monomerfixed on the surface of the support, and the characteristics of the filmor coating, for example its thickness, stabilize. This mechanism is veryprobably at the source of the autoregulation of the thickness of thefilm that is observed by using the method of the present invention.

In practice, it is necessary to examine the operational conditions bywhich the higher end of the coverage rate of the surfaces is reached;that is, the situation of blockage and autoregulaton mentioned above andto what degree it can be expected with a certain tolerance regarding theohmic drops; in other words over a range of potentials.

Determination of the potential V_(bloc); that is, the minimal level ofpotential used as the stop potential under voltammetric conditions or asthe plateau potential in polarization by multiple segment for graftingand growth of a given organic polymer film according to them method ofthe present invention is realized using plotting of a graph giving thethickness of the electro-grafted organic polymer film as a function ofthe number of scans.

The appended FIG. 5 represents one such graph. In this figure, Tr (%)represents the percentage of transmission at the wavelength ofabsorption of the vibrator of the CN nitrile bonds of the electrograftedfilm (measured on infrared reflection spectroscopy, IRRAS) and Nrepresents the number of scans. The nomogram of FIG. 4 shows that this %of transmission is connected with the thickness of the electro-graftedfilm, measured independently by profilometry.

In this figure, V_(bloc) is the level of potential form which the curvesproducing the thickness of the electro-grafted film as a function of thenumber of scans are all superimposed.

In this example, in which the monomer precursor is methacrylonitrile,V_(bloc) is from −2.3 to −2.5 V (AG⁺/Ag), preferably around −2.4V/(Ag⁺/Ag).

Electro-grafting according to the present invention can be tracked bymeans of a typical voltammogram for the electro-grafting reactions asrepresented in the appended FIG. 2. This figure represents the current(in mA) as a function of potential (−E) at which the surface is exposed,this potential being marked relative to a silver reference electrode.The horizontal arrows indicate the potential scans. V represents thepotential at a time t, during scans.

This votammogramme is characterized by the following criticalpotentials:

For V<V_(s), “V_(s)” being the threshold potential, there is almost noelectrical current. The system is quasi-stationary from the point ofview of electronic exchanges and the surface remains unchanged.

For V_(s)<V<V_(g), “V_(g)” being the potential starting at whichgrafting on the surface of the monomer precursor appears, the current isdue essentially to the mechanism of the appended FIG. 1 b) and noelectro-grafted film is observed.

For V_(g)<V<V_(bloc), “V_(bloc)” being the potential of blockage, partof the current is to the electro-grafted film, and part is used in theformation of a polymer in solution. This corresponds to the diagram ofFIG. 1 a). The blockage potential is rather delicate to preciselydefine, but it can be considered that it is greater than the peakpotential, V_(pic). It is when the potential is situated in this zonethat grafting and thus augmentation of the coverage rate of the surface,occurs. For example, voltammetric scans can be carried out between aninitial potential “V_(i)” and a final potential “V_(fin)”. It is whenV_(fin)>V_(bloc) that the repeated scans make it possible to reach theblockage process according to the present invention. In anotherembodiment of the process of the present invention, the same result canbe obtained with multiple-sectors of potential.

For V>V_(bloc), the electrical current is limited either by diffusion orby the reaction of polymerization in solution. In general fashion,sensitive modification of the thickness of the film obtained in thiszone of potential is no longer observed—beyond a minimal number ofscans—because the blockage of the grafting reaction has occurred.

In FIG. 6, it is observed that whatever the stoppage potential V_(fin),the thickness of the film electro-grafting is marked out as a functionof the number of scans: The curves giving the thickness as a function ofthe number of scans having an asymptote, that permits estimating thelimit thickness that can be attained by using an end potential V_(fin)given as a synthesis parameter.

FIG. 6 represents the % of transmittance of the nitrile labelcorresponding to these limit thicknesses as a function of the endpotential V_(fin) of the voltammetric scans utilized according to thedata of FIG. 5. It is observed that there is a cathodic potential,V_(bloc), beyond which the thickness of the electro-grafted film isindependent of the potential V_(fin) utilized: this observation servesto define the blockage potential, V_(bloc).

Thus, according to the present invention, a succession of voltammetricscans with a “flat” final potential V_(fin), for example under theeffect of a parasitic ohmic drop or due to a complex surface, will nothave an influence on the quality of the grafted organic film, nparticular on its surface, such that the effective V_(fin) potentialwill be greater than V_(bloc) everywhere on the surface on which thefilm must be grafted.

Thus, when the electro-grafting by voltammetric scanning is doneaccording to the method of the present invention, for example, using aninitial potential of −0.7 V (Ag⁺/Ag) at a scanning speed of 100 mV/s ona gold surface of a 1.5 mol/l solution of methacrylonitrile in thedimethyl formamide (DMF) in the presence of 10⁻² mol/l of perchlorate oftetraethyl ammonium (TEAP): one has V_(s) (−1.6 V/(Ag⁺/Ag),V_(pic)=−2.25 V/(Ag⁺/Ag), V_(g) (−2.2 V/(Ag⁺/Ag), V_(bloc) (−2.3V/(Ag⁺/Ag). The currents observed are in the order of milliamperes.

According to the method of the present invention, the maximal potentialof each potential scan must, in consequence, be attained, at minimum, atevery point of the surface on which the polymer film is grafted in orderto obtain a uniform film thickness.

We shall consider a first and a second gold surface connectedelectrically in series but separated by a 100 kΩ resistor, one of whichis connected to a potentiostat. It is a complex surface in the contextof the present invention. For currents of the order of the milliampere,the ohmic drop between the two gold surfaces is on the order of 100 mV.Thus, if the first surface connected to the potentiostat is, forexample, polarized up to −2.35 V/(Ag⁺/Ag) or if, under voltammetricconditions, the final potential of the scan is V_(fin)=−2.35 V/(Ag⁺/Ag),the second surface, connected to the first, will be only a potential ofapproximately −2.25 V/(Ag⁺/Ag) or very close to the grafting potentialV_(g). Even if after several scans, the inventors observed anelectro-grafted film less thick over the first surface directlyconnected to the potentiostat than on the second surface.

If the first surface connected to the potentiostat is brought to apotential of −2.6 V/(Ag⁺/Ag) or if, under voltammetric conditions, thefinal potential of scanning is V_(fin)=−2.6 V/(Ag⁺/Ag), the potential towhich the second surface is brought −2.5 V/(Ag⁺/Ag) is greater than theblockage potential. Thus, the maximal potential of each scan is greaterthan V_(bloc) on every point of this complex surface. For a number ofsufficient voltammetric scans, an electro-grafted film is obtained onthe two surfaces of identical thicknesses. The film obtained has auniform thickness of approximately 90 nanometers for 10 scans over allof the surface.

The values of V_(g) and V_(bloc) are difficult to indicate with greatprecision for a generalization. In fact, for V_(g) it is the value atwhich one is capable of demonstrating an electro-grafted film: thisestimation can vary also as a function of the means of analyzing thesurface used to obtain this detection, because the sensitivity of aninfrared reflection-absorption spectroscopy apparatus, for example, isnot the same as that of a photoelectron X spectroscopy apparatus.

As indicated in the exemplary embodiments below, the error levels areassociated with the thicknesses according to the measurement methodutilized; for example, by profilometry or ellipsometry, which combinedwith the allowed tolerance regarding the notion of “uniformity ofthickness” can lead to a certain inaccuracy relative to the value of thesaturation potential V_(bloc). It must be noted that the imprecision isnot intrinsic to the procedure but at the discretion of the operatorregarding the fineness of his control.

The person skilled in the art will know how easily to adapt the processof the present invention by following the indications provided in thisdescription for obtaining a uniform film thickness over the entiresurface.

The present inventors have also shown that the electro-initiatedreactions according to the method of the invention are also reactionshaving a threshold of imprecision of potential trigger. They havediligently taken part of this observation to obtain, over two distantzones, organic depositions of different thicknesses simultaneouslycontrolled by electrically connecting these two zones in series using acarefully chosen impedance. For example, two gold spots can be producedon a single wafer by a 100 kΩ electrical resistor. By using, under theaforementioned voltammetric conditions, a stoppage potential ofV_(fin)=2.30 V/(Ag⁺/Ag), the surface connected to the potentiostat “willbe” a potential of −2.30 V/(Ag⁺/Ag), whilst the surface linked to theformer will be only a potential of −2.20 V/(Ag⁺/Ag): according to FIG.5, it will be seen that a film producing a transmittance of 1.5% will beformed on the surface connected to the potentiostat, while the filmformed on the surface linked to the first via the resistor will have athickness of 0.9%.

In contrast, when one intends to obtain two thicknesses produced at twodifferent places, the stoppage potential is chosen as being that givingthe greatest of the two thicknesses (FIG. 5), and the data of FIG. 5make it possible to determine the potential that should “see” the zoneon which the second thickness is to be deposited. Knowing the electricalcurrent connected to the electro-grafting reaction, the impedance thatmust be incorporated between the two zones of the surface, upon whichthe different thicknesses are desired, can thus be determined.

BRIEF DESCRIPTION OF THE FIGURES

The FIGS. 1 a) and b) are a diagrammatic illustration of theelectrochemical reactions in play in the method of the present inventionwhen the monomer precursor of the organic polymer is acrylonitrile.

FIG. 2 is a typical voltammogram accompanying an electro-graftingreaction of an activated vinyl monomer. The potentials are to be takeninto account in absolute values; they are in fact negative for acathodic polarization and positive for an anodic polarization.

FIG. 3 represents the voltammograms obtained on gold at 100 mV/s of thefirst and tenth scan of a solution of 2.5 mol/l f methacrylonitrile inDMF in the presence of TEAP.

FIG. 4 is a plotting showing the thickness of the polymethacrylonitrile(PMAN), measured by profilometry as a function of the transmittance ofthe band of the nitrile group (Tr (%)) of the PMAN at 2,200 cm⁻¹obtained by IRRAS.

FIG. 5 is a diagrammatic representation of the thickness of thepolymethacrylonitrile (PMAN) film electro-grafted onto gold andvoltammetric conditions as a factor of the number of scans, fordifferent values of stoppage potential.

FIG. 6: is a diagrammatical representation of the thickness of the filmof polymethacrylonitrile (PMAN) (evaluated via the transmittance of thenitrile label in IRRAS) the electro-grafted on gold under voltammetricconditions as a function of the stoppage potential V_(fin) utilized in aoperational protocol under voltammetric conditions. The transmittancesutilised are those of the asymptotes observed in the protocol of FIG. 5.

FIG. 7: is a diagrammatic representation viewed from aboe the bladesused: If epitaxy then doped n on an SOI substrate, then a sub-layer oftitanium/nickel on the moiety of the lame, and finally deposited in goldunder vacuum on the adhesion sub-layer.

FIG. 8: is an IRRAS spectrum of the Poly 4-vinyl pyrridine (P4VP)obtained by electro-grafting on gold, for different stoppage potentialsV_(fin).

FIG. 9: is a graphic representation of the variation of thicknessmeasured by profilometry of P4VP films obtained by 20 voltammetric scansbetween the potential of equilibrium and different stoppage potentialV_(fin).

FIG. 10: is an IRRAS spectrum of the Poly 4-vinyl pyrridine (P4VP)obtained by electro-grafting on gold, for different stoppage potentialsV_(fin).

FIG. 11: is a graphic representation of the variation of thicknessmeasured by profilometry of PBUMA films obtained by 20 voltammetricscans between the potential of equilibrium and different stoppagepotential V_(fin).

In the figures: “S” represents the surface; “Vp” represents the peakpotential; “V_(g)” representing the threshold potential from which theelectro-grafting appears on the surface; “VS” is the threshold potentialon—beyond which there is no current on the surface; “Vi” is the initialscanning potential; “B” indicates the scanning on potential; “Vb”indicates Vbloc, a blockage potential of blockage; “Vf” indicates thefinal potential of blockage; “Vf” indicates the final surface potentialof scanning on surface scanning potential; “C” the current in MA; “I”the intensity in μA; “e” the thickness in nm; “Tr” the transmission in%; “N” the number of cycles or potential scan number; “NO” the wavenumber of in cm⁻¹.

EXAMPLES Example 1 Critical Potential Qualification MethacrylonitrileElectro-Grafting on Gold

Glass plates, such as microscope slides, coated with a gold depositobtained by evaporation on a chromium underlayer are used as workingelectrodes. A standard three-electrode assembly is used, with a silverelectrode used as the reference electrode.

The three electrodes are immersed in a 2.5 mol/l methacrylonitrilesolution in dimethyl formamide (DMF) in the presence of 10⁻² mol/l oftetraethyl ammonium perchlorate (TEAP). Using a potentiostat, a linear100 mV/s potential sweep from a potential V_(i)=−0.6 V/(Ag⁺/Ag) to apotential V_(fin) and back is applied to the working electrodes.

FIG. 3 appended shows the voltammogram of the first sweep 1B on whichthe system moves, and that of the tenth sweep 10B.

Each plate, used as a working electrode, is subjected to, for a givenpotential V_(fin), a number of potential sweeps between V_(i) andV_(fin). The plate is removed, rinsed with DMF and dried. The thicknessof the electro-grafted film obtained is then measured by means of itsinfrared reflection spectrum (IRRAS), using the scale represented inFIG. 4 appended.

The same work is carried out on several series of plates obtained withstopping potentials V_(fin) of −2.10; −2.15; −2.20; −2.25; −2.30; −2.35and −2.40 V/(Ag⁺/Ag).

The curve giving the thickness of the films obtained on the variousplates as a function of the number of sweeps for these differentstopping potentials or maximum sweep potentials, is given in FIG. 5appended.

It is observed that the curves all coincide for V_(fin) (−2.30V/(Ag⁺/Ag) which thus corresponds to the saturation or blockingpotential within the scope of the thickness measurements made (see FIG.6). This potential value corresponds to V_(bloc) within the scope of thepresent invention for this organic polymer film.

It is also observed that for a stopping potential greater than thesaturation potential, the thickness reproducibility is of very goodquality, since the variation observed is of the order of 5 to 10 nm inthis instance.

This example demonstrates that a homogeneous film thickness can beobtained by means of the method according to the present invention usingthe potential V_(bloc) determined on the graph in FIG. 5 appended as themaximum potential of each sweep.

Example 2 Obtaining Electro-Grafted Films of Similar Thicknesses on TwoTerminals Via a Null and Non-Null Impedance, Respectively

The purpose of this example is to demonstrate that it is possible tocheck the thickness of an electro-grafted film despite the presence ofan ohmic drop between the potentiostat and the conductive surface onwhich the electro-grafting takes place.

The surfaces selected to illustrate this example are composite platesobtained from an SOI (SiO₂) substrate on which silicon is deposited bymeans of epitaxy, followed by ionic doping n, and gold deposition, in avacuum, over half of the plate, having previously deposited atitanium/nickel adhesion underlayer of a few nanometres.

The samples obtained are represented schematically in FIG. 6 appended.They consist of complex surfaces within the scope of the presentinvention.

These samples are used as working electrodes in a three-electrodeelectrochemical cell.

Synthesis is carried out according to the following protocol:

Solution: Methacrylonitrile (40%) in DMF, in the presence of 5.10⁻² M oftetraethyl-ammonium perchlorate (TEAP);

Cell: Teflon, without separate compartments, volume 13 ml;

Counter-electrode: 10.4 cm² Pt sheet;

Reference electrode: Ag/AgClO₄;

Working electrode surface area: 6.6 cm²;

Electrochemistry: N cycles, voltammetric conditions: 100 mV/s:V_(on)=−0.6 V/(Ag⁺/Ag) to V_(fin) (variable). The entire plate isimmersed in the synthesis medium. The electrical contact (crocodileclip) is carried out either on gold or on silicon;

Electrode rinsing: 15 minutes in ultrasound in acetone +15 minutes inultrasound in water.

Three additional tests were carried out:

(i) the electrical contact of the working electrode is taken on the goldterminal. The plate is treated with voltammetric sweeps, up to astopping potential of −2.4 V/(Ag⁺/Ag);

(ii) the electrical contact of the working electrode is taken on thesilicon terminal. The plate is treated with voltammetric sweeps, up to astopping potential of −2.4 V/(Ag⁺/Ag);

(iii) the electrical contact of the working electrode is taken on thesilicon terminal. The plate is treated with voltammetric sweeps, up to astopping potential of −2.6 V/(Ag⁺/Ag).

The thicknesses in (mm) of the polymethacrylonitrile coatings obtainedon the gold are measured by means of profilometry, as a function of thetransmittance (in %) of the band of nitrile groups of PMAN at 2200 cm⁻¹obtained by IRRAS. The results are represented in FIG. 4 appended.

Table 1 below summarises the operating conditions and thicknesses of thepolyacrylonitrile coatings obtained on the gold terminal of samples ofthe type represented in FIG. 6 according to whether the electricalcontact (crocodile clip) is taken on the gold terminal i or on thesilicon terminal ii and iii.

TABLE 1 V_(fin) Thickness Experiment N (V/(Ag⁺/Ag)) (nm) (i) 50 −2.4 48(5 (ii) 50 −2.4 10 (5 (iii) 50 −2.6 38 (5

Under the selected operating conditions, the blocking potential V_(bloc)is substantially the same as that in example 1.

The conditions in experiments i and ii are such that the stoppingpotential is similar to the blocking potential, but in experiment ii,there is an ohmic drop between the potentiostat terminal and the goldterminal, to the extent that the potential actually “experienced” by thegold terminal is less than the blocking potential: the thickness of thepolymethacrylonitrile film is substantially less than that obtained inexperiment i.

In experiment iii, experiment ii is reproduced, but this time byapplying a more cathodic stopping potential than the blocking potential,it is observed that a thickness comparable to that obtained inexperiment i is achieved on the gold terminal.

Therefore, the potential drop caused by the silicon terminal, which wasnot measured, can be estimated at approximately 200 mV.

The results demonstrate that it is indeed the value of the workingelectrode potential that, for a sufficient number of voltammetricsweeps, makes it possible to check the thickness of the electro-graftedfilm.

In addition, the films obtained are of a homogeneous thickness on eachsurface according to the present invention, is greater than V_(bloc) atall points of the surface, i.e., in view of the abovementioned (ohmic)potential drop, when the potential in case ii on the electrical contactis, in absolute values, greater than or equal to V_(bloc)+200 mV.Indeed, the potential on the gold terminal is then greater than or equalto V_(bloc), which ensures a uniform film thickness on this complexsurface within the scope of the present invention.

Example 3 Qualification of V_(BLOC) Potential for 4-Vinyl PyridineElectro-Grafting on Gold

Glass plates coated with gold as described in example No. 1 are used asworking electrodes.

A standard three-electrode assembly is used, with a silver electrodeused as the reference electrode. The three electrodes are immersed in a30% 4-vinyl pyridine solution distilled in commercial DMF in thepresence 5.10⁻² mol/l tetramethyl ammonium perchlorate (TMAP).

Using a potentiostat, 20 cycles of a linear 100 mV/s potential sweepfrom an initial potential V_(i) equal to the equilibrium potentialV_(fin) and back is applied to the working electrodes. This number ofcycles was chosen by the inventors because it corresponds to the minimalnumber of cycles, for this monomer and under these operating conditions,which makes it possible to achieve the asymptote of the curve giving thethickness as a function of the number of cycles under voltammetricconditions.

Four potentials V_(fin) are taken into consideration: −2.4, −2.5, −2.6and −2.7 V/(Ag⁺/Ag). Under operating conditions, the peak potential isequal to −2.4 V/(Ag⁺/Ag).

Each plate is removed, rinsed with DMF and dried in a nitrogen stream.The thickness of the electro-grafted film obtained is then measured bymeans of its IRRAS infrared reflection spectrum represented in FIG. 8appended, using a suitable scale. These thicknesses are also measuredindependently by means of profilometry, and it is observed that theresults of both types of measurements are consistent.

The curve giving the thickness of the films obtained on the variousplates as a function of the number of sweeps for these differentstopping potentials is given in FIG. 9 appended.

A thickness asymptote is observed for V_(fin) (−2.60 V/(Ag⁺/Ag), whichthus corresponds to the saturation potential V_(bloc) for 4-vinylpyridine on gold within the scope of the measurements made. It isobserved that for a stopping potential greater than the saturationpotential, the thickness reproducibility is of very good quality, sincethe variation observed is of the order of 5 to 10 nm.

Example 4 Qualification of V_(BLOC) Potential for Butyl Methacrylate(BuMA) Electro-Grafting on Gold

Glass plates coated with gold as described in example No. 1 are used asworking electrodes.

A standard three-electrode assembly is used, with a silver electrodeused as the reference electrode. The three electrodes are immersed in a30% butyl methacrylate (BuMA) solution in commercial DMF in the presenceof 5.10⁻² mol/l tetramethyl ammonium perchlorate (TMAP).

Using a potentiostat, 20 cycles of a linear 100 mV/s potential sweepfrom an initial potential Vi equal to the equilibrium potential V_(fin)and back are applied to the working electrodes. This number of cycleswas chosen by the inventors because it corresponds to the minimal numberof cycles, for this monomer and under these operating conditions, whichmakes it possible to achieve the asymptote of the curve giving thethickness as a function of the number of cycles under voltammetricconditions.

Four potentials V_(fin) are taken into consideration: −2.4, −2.7, and−2.9 V/(Ag⁺/Ag). Under operating conditions, the peak potential is equalto −2.73 V/(Ag⁺/Ag).

Each plate is removed, rinsed with DMF and dried in a nitrogen stream.The thickness of the electro-grafted film obtained is then measured bymeans of its IRRAS infrared reflection spectrum represented in FIG. 10appended, using a suitable scale. These thicknesses are also measuredindependently by means of profilometry, and it is observed that theresults of both types of measurements are consistent.

The curve giving the thickness of the films obtained on the variousplates as a function of the number of sweeps for these differentstopping potentials is given in FIG. 11.

A thickness asymptote is observed for V_(fin) (−2.60 V/(Ag⁺/Ag), whichthus corresponds to the saturation potential V_(bloc) for 4-vinylpyridine on gold within the scope of the measurements made according tothe method of the present invention. It is observed that for a stoppingpotential greater than the saturation potential, the thicknessreproducibility is of very good quality, since the variation observed isof the order of 5 to 10 nm.

1. Method for cladding a simple or complex surface, electricallyconducting or semiconducting, by means of an organic film from at leastone precursor of said organic film, characterised in that the claddingof the surface by the organic film is carried out by electro-initiatedgrafting of said, at least one, precursor of said surface by applying atleast one potential sweep on this surface carried out in such a way thatat any point of said surface the maximum potential of each potentialsweep, in absolute value and relative to a reference electrode, isgreater than or equal to the value of the potential (V_(bloc)) fromwhich the curves of a graph expressing the quantity of electro-graftedprecursor on a surface identical to said surface in function of thenumber of potential sweeps are all superposed and independent of thisV_(bloc) potential.
 2. Method according to claim 1, wherein the organicfilm is an organic polymer film and the precursor is an electro-activemonomer precursor of said organic polymer film.
 3. Method according toclaim 2, wherein the monomer precursor of the organic polymer is avinylic monomer or a mixture of vinylic monomers.
 4. Method according toclaim 2, wherein the, at least one, monomer precursor of the organicpolymer being a vinylic monomer, it is chosen from amongst the groupconstituted of vinylic monomers such as acrylonitrile,methacrylonitrile, methyl methacrylate, ethyl methacrylate, butylmethacrylate, propyl methacrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate, glycidyl methacrylate, the acrylamides andin particular the amino-ethyl, propyl, butyl, pentyl and hexylmethacrylamides, the cyanoacrylates, di-methacrylate polyethyleneglycol, acrylic acid, methacrylic acid, styrene, parachloro-styrene,N-vinyl pyrrolidine, 4-vinyl pyridine, the vinyl halides, acryloylchloride, methacryloyl chloride, and their derivatives.
 5. Methodaccording to claim 2, wherein the electro-initiated polymerisationconsists of electro-grafting of cyclic monomers cleavable bynucleophilic or electrophilic attack.
 6. Method according to claim 2,wherein the electro-initiated polymerisation consists ofelectro-grafting of salts of diazonium, sulfonium, phosphonium oriodonium.
 7. Method according to claim 2, wherein the electro-initiatedpolymerisation is initiated by salts of diazonium, sulfonium,phosphonium or iodonium, in the presence of polymerisable monomers byfree radical means such as vinylic monomers or cleavable cyclicmolecules.
 8. Method according to claim 1, wherein the precursor is avinylic monomer coupled to a molecule or a macromolecule chosen amongstthe group constituted of a polymer such as polyethylene glycol; of anitrogenous base or a derived nitrogenous base; of an oligonucleotide;of a peptide; of a fatty acid; of a glucide; of a polysaccharide,modified or not; of cellulose and its derivatives and of chitosan andits derivatives.
 9. Method according to claim 2, wherein N potentialsweeps are applied, N being a whole positive number, with 1≦N≦15. 10.Method according to claim 2, wherein the potential sweep is avoltammetric or multi-slot sweep.
 11. Method according to claim 2, inwhich the precursor monomer being methacrylonitrile, V_(bloc) is −2.3 to−2.5 V (Ag⁺/Ag).
 12. Cladding obtained by the method according toclaim
 1. 13. Utilisation of a method according to claim 1, for theorganic functioning of a surface.
 14. Utilisation of a method accordingto claim 1, for the manufacture of a biochip or a sensor. 15.Utilisation of a method according to claim 1 for producing an insulatinglayer or a barrier layer in a micro-system.
 16. Utilisation of a methodaccording to claim 1 to produce an insulating layer or a barrier layerfor microelectronics.